I. Field of the Invention
The invention relates to asphalt compositions containing asphalt and sulfur.
II. Description of the Prior Art
Road building has been a source of inspiration and aggravation for civilization for millennia. Ancient Rome built roads that would last for centuries, but which required an enormous investment in labor and materials and a high level of sophistication. In the 1830's, travelers prized the opportunity to travel on short stretches of macadam roads made of three layers of graded stone, the top layer of which contained some bituminous or asphaltic binder.
The need to reduce costs and use locally available materials led to wooden plank roads, which were, in turn, replaced with asphalt roads. By the early 20 .sup.th century, asphalt roads, mixtures of gravel and asphaltic petroleum fractions, were the dominant roads. Many times an asphalt gravel mix was simply poured on a minimally prepared surface. Such roads could be built cheaply and quickly with unsophisticated labor, but deteriorated rapidly.
Roman roads lasted for centuries, but took decades to build. Macadam roads, built according to the original rigid specifications of 1830, lasted for decades, but took months or years to build. Many asphalt roads were built in weeks or months only to deteriorate after a few years. Asphalt roads that deteriorate rapidly do so because of lack of surface preparation and drainage, poor construction practices and poor quality materials.
Asphalt roads have come full circle. The early macadam roads were prized for their quality and durability. Many modern macadam roads suffer a largely undeserved poor reputation for quality.
There is a renewed interest in building quality roads with asphalt as civil engineers and municipalities realize that better asphalt roads are possible by using better surface preparation, construction practices; and better asphalts.
Some improvements in asphalt properties were achieved by selecting the starting crude petroleum, or control of the refinery processing steps used to make the asphalt Unfortunately, there are many crudes which do not make good asphalts. There are only a limited number of steps which can be taken to control the refining process to make better asphalt.
The next step taken by the industry was to modify the asphalt. Air blowing makes asphalts harder. Fluxing agents or diluent oils are sometimes used to soften the asphalt.
Marked changes in asphalt properties can be achieved with sulfur, either added to neat asphalt or when added as a cross-linking agent to treat a mixture of asphalt and polymer.
The conventional methods of adding sulfur (dumping loose powder or bags of powdered sulfur on top of molten asphalt in a mix tank) had some problems. Of significant concern is safety. There exists the potential for fire and explosion hazards, caused by having a potentially large cloud of hot and flammable sulfur dust. Sulfur dust is by its very nature considered explosive.
It was also difficult in a commercial facility to rapidly and completely mix powdered sulfur with asphalt. Some parts of the asphalt saw too much sulfur too rapidly while other parts of the blend were sulfur deficient. Such an approach, led to over cross-linking parts of the blend, forming lumps of super-vulcanized materials which were not compatible with the rest of the blend.
Some work has been conducted on adding molten sulfur and mixing the liquid sulfur with liquid asphalt. A somewhat similar approach was forming a "mother liquor", or blend of sulfur, oil, and polymer, and adding this to the asphalt and then mixing. These approaches can help, but were not the ultimate solution.
Having provided a relatively readable overview of road building from ancient Roman times to the present, I will now provide some detailed discussion of the prior art